[求助]蓝皮书P312--95

1.        Studies of the human “sleep-wake cycle” have practical relevance for matters ranging from duty assignments in nuclear submarines and air-traffic control towers to the staff of shifts in 24-hour factories.

(A) to the staff of

(B) to those who staff

(C) to the staffing of

(D) and staffing（C）

(E) and the staff of

答案: C   为什么不是A?